Potassium cobalt hexacyanoferrate is known in the art as an effective adsorbent for cesium ions and has found important use in removing radioactive cesium ions from nuclear wastes via an ion exchange process. The method has been described, for example, in U.S. Pat. No. 3,296,123; in "A Review of Information on Ferrocyanate Solids for Removal of Cesium from Solutions," P. A. Haas, Sep. Sci. Technol. 28 (17-18), 2479-2508 (1993); and in "Evaluation of Selected Ion Exchangers for the Removal of Cesium and Strontium from MVST W-25 Supernate," J. L. Collins, et al., ORNL/TM-12938, April 1995.
Potassium cobalt hexacyanoferrate, hereinafter referred to as "KCOHEX," has typically been prepared by a method in granular form as described in U.S. Pat. No.3,296,123, wherein an aqueous acidic solution of potassium ferrocyanide is slowly mixed with an aqueous solution of cobalt nitrate, as shown in Formula I. EQU potassium ferrocyanide+cobalt nitrate.fwdarw.KCOHEX+potassium nitrate
Water is removed by centrifugation, the wet cake is washed with water, then dried in an oven to form a dried solid mass. The solid mass is ground and sized, and particles of from about 150 micrometers to about 450 micrometers are packed into columns for subsequent exposure to radioactive wastes containing, in particular, Cesium-137.
The method of preparing particulate KCOHEX and other hexacyanoferrates suffers from two significant drawbacks. First, grinding the dried solids must be done carefully so as to minimize formation of unusable fines. Second, since a wide range of particle sizes results from grinding, the particulate must be sized through sieves. These operations are time-consuming and inevitably cause loss of product.
As described in the references noted above, sized KCOHEX is then loaded into columns in order to remove cesium from radioactive waste solutions.
Spray-drying of solid materials is a method known in the art for preparation of useful solids. See, for example, Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed., John Wiley & Sons, New York, 1993; Vol. 8, p. 475-519, particularly pp. 505-508; and C. Strumillo and T. Kudra, "Drying: Principles, Applications and Design," Gordon and Breach, New York, 1986, pp. 352-359. However, in all of the reports of preparation and use of KCOHEX, a spray drying process has not been described.